Reading every single word of a textbook, is not an efficient use of time. You will benefit far more, if you read the content that is most relevant to you, from a handful of textbooks, rather than everything from one book.
Furthermore, if you start reading every single word of a thick textbook, straight away, then you are likely to get bored halfway through and give up!
Before diving into the details, skim through the textbook to get a better overview of what it contains – look at the main chapter headings, subheadings and any highlighted or emboldened text. Once you have a better understanding of the structure and main ideas presented in the book, you can then start to delve into the parts that are most relevant to your learning.